1. Field of the Invention
The present invention relates to an automatic tracing method for a welding torch in a lap joint welding operation of a consumable electrode type arc welding robot.
2. Description of the Related Art
In the past, as a method for automatically controlling the lap joint welding of a welding torch 4 of a consumable electrode type arc welding robot as shown in FIG. 1, that is, in which an upper plate (web) 2 laps on a lower plate (flange) 3 to define a weld groove line 6, there has been proposed an automatic control method which utilizes the fact that a weld current value varies according to a distance between the welding torch 4 and the material to be welded.
The tracing of the weld groove line 6 according to the aforesaid method is accomplished by oscillating the welding torch widthwise relative the length of the groove, integrating weld currents at both ends of the oscillation and comparing the thus integrated current values to determine if they are equal to each other, and moving the oscillation center 5 of the welding torch 4 towards the oscillation end having the smaller of the integrated current values if the values are unequal.
Further, tracing relative the vertical direction of the weld groove line 6 is accomplished by comparing an average value of the integrated current values of the weld current with an adequate preset current value, and if the average weld current value is larger than the preset current value, the distance between the welding torch and the material to be welded is deemed to be too short and therefore an upward correction is made, whereas if the average integrated value of the weld current is smaller than the preset current value, the distance between the welding torch and the material to be welded is deemed to be too long and therefore, a downward correction is made.
However, in the above method of comparing the integrated current values at both oscillation ends, then the lap joint welding is as shown in FIG. 1, the wall 2 of the upper plate (web) 1 is relatively small, and the wall 2 may become inclined or melted at the lower plate (flange) 3 side to lose a corner thereof. When the oscillation center 5 of the welding torch 4 is deviated from the weld groove line 6 towards the upper plate 1 the projected length of a wire 7 on the upper plate 1 side of oscillation is longer than the projected length of a wire on the lower plate 3 side of oscillation as indicated by the distance "m". Therefore, the integrated current value on the upper plate 1 side of oscillation is smaller than the integrated current value on the lower plate 3 side of oscillation, resulting in the issuance of a command for correction towards the upper plate 1 side. As a result, an opposite correction is made in that the oscillation center should be corrected towards the lower plate 3 side to effect the automatic tracing along the weld groove line 6, resulting in an increase in the deviation from the weld groove line 6 towards the upper plate 1.
When once departed towards the upper plate 1, the length of the wire 7 at the upper plate 1 side of oscillation necessarily becomes long as shown in FIG. 2, and therefore, the weld integrated current value at the oscillation end of the upper plate 1 is small. As a consequence, a further correction towards the upper plate 1 is effected, and the oscillation center 5 continues to be moved away from the weld groove line 6.
As a result, an operating state results in which the center of oscillation departs from the weld line in the automatic tracing method. Due to the occurrence of this phenomenon, the wall 2 of the material to be welded must be high, as in a T-shape fillet, to accomplish excellent automatic tracing, but if the material to be welded has a wall 2 which is liable to be melted, as in a lap fillet, the use thereof is difficult.